<body>
  <div id="app">
    <input type="text" id="txt">
    <p id="show"></p>
  </div>
</body>
<script type="text/javascript">
  var obj = {}
  Object.defineProperty(obj, 'txt', {
    get: function () {
      return obj
    },
    set: function (newValue) {
      document.getElementById('txt').value = newValue
      document.getElementById('show').innerHTML = newValue
    }
  })
  document.addEventListener('input', function (e) {
    obj.txt = e.target.value
  })
</script>